Hydrocephalus was induced in 12-day old rats by the cisternal infusion of a concentrated kaolin suspension. The animals were killed at day 20 and the ependymal lining of all the ventricles prepared for scanning electron microscopy. The dilation of the ventricles was moderate to gross in all cases. The ependyma of the lateral ventricles was similar in both control and experimental animals. Ependymal damage was present in six out of the twelve hydrocephalic rats. Two had fibres visible on the ependymal surface. Four had tears covered with small round cells, believed to be responsible for the repair of the ependyma. The third ventricle, cerebral aqueduct and fourth ventricle enlarged by incorporating folds of ependyma, present in control animals, into the ventricular walls. The circumventricular organs present in the third and fourth ventricles were not damaged by the dilation of the ventricles, even in severe hydrocephalus.